This miraculous feat only occurs when the jellyfish has been harmed or is on the brink of starvation. In response, it winds back its biological clock to the time when it was a small larva.

Jellyfish can produce offspring both sexually ... for the comblike rows of tiny paddles they use to swim, they tend to be small, delicate, and hard to study. They come in an array of weird body ...

These are typically smaller than scyphozoan jellyfish. The polyp stage of their lives is usually more conspicuous than the small adult jellies. However, not all hydrozoans are considered jellyfish.

Jellyfish are known for drifting to and fro ... forth across the lake they mix its waters—and churn the nutrients and small organisms that form the base of the food chain.

This phyllosoma, a lobster larva, is just 1.2 centimetres across. With a flattened body and eyes on stalks, its spindly legs grip the empty bell of a small dead jellyfish, a mauve stinger. When alive, ...